mv include/ruby/{3,impl}

Devs do not love "3".

include/ruby/impl/intern/select.h

@@ -0,0 +1,52 @@
+#ifndef RUBY3_INTERN_SELECT_H                        /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY3_INTERN_SELECT_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    Symbols   prefixed   with   either  `RUBY3`   or   `ruby3`   are
+ *             implementation details.   Don't take  them as canon.  They could
+ *             rapidly appear then vanish.  The name (path) of this header file
+ *             is also an  implementation detail.  Do not expect  it to persist
+ *             at the place it is now.  Developers are free to move it anywhere
+ *             anytime at will.
+ * @note       To  ruby-core:  remember  that   this  header  can  be  possibly
+ *             recursively included  from extension  libraries written  in C++.
+ *             Do not  expect for  instance `__VA_ARGS__` is  always available.
+ *             We assume C99  for ruby itself but we don't  assume languages of
+ *             extension libraries. They could be written in C++98.
+ * @brief      Public APIs to provide ::rb_fd_select().
+ * @note       Functions  and  structs defined  in  this  header file  are  not
+ *             necessarily ruby-specific.  They don't need ::VALUE etc.
+ */
+#include "ruby/3/config.h"
+
+#ifdef HAVE_SYS_TYPES_H
+# include <sys/types.h>         /* for NFDBITS (BSD Net/2) */
+#endif
+
+#include "ruby/3/dllexport.h"
+
+/* thread.c */
+#if defined(NFDBITS) && defined(HAVE_RB_FD_INIT)
+# include "ruby/3/intern/select/largesize.h"
+#elif defined(_WIN32)
+# include "ruby/3/intern/select/win32.h"
+# define rb_fd_resize(n, f) ((void)(f))
+#else
+# include "ruby/3/intern/select/posix.h"
+# define rb_fd_resize(n, f) ((void)(f))
+#endif
+
+RUBY3_SYMBOL_EXPORT_BEGIN()
+
+struct timeval;
+
+int rb_thread_fd_select(int, rb_fdset_t *, rb_fdset_t *, rb_fdset_t *, struct timeval *);
+
+RUBY3_SYMBOL_EXPORT_END()
+
+#endif /* RUBY3_INTERN_SELECT_H */